Transducer



y 8, 1952 E. o. JOHNSON 2,60

TRANSDUCER Filed Sept. 20, 1950 10 25% Z I Z i Il i 16 i l i as INVEN 1mg a iaiiism ATTORNEY Patented July 8, 1952 TRANSDUCER Edward 0. Johnson, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 20, 1950, Serial No. 185,748

7 Claims. 1

This invention relates to improved transducers, such as microphones or phonograph pickups, for converting mechanical movements into electrical signals. More particularly it relates to a novel kind of transducer which includes a gas discharge valve arrang'edtherein to afford great power sensitivity and a very low output impedance.

Substantially all of the transducers of the prior art, including those of the vacuum tube types, have been incapable of delivering (or controlling) more than extremely small amounts of power. Because of this most sound transmitting and sound reproducing systems have included audio amplifiers between their transducers and reproducers. The presence of amplifiers in these systems has made them more costly and has been largely to blame for what may be called poor overall conversion efiiciencies. The latter fact is due to a multiplication of the individual losses entailed in the numerous energy conversions required in the amplifiers, such as in the conversion of heater current into thermionic emission; in the conversion of A. C. line power into suitable direct potentials for amplifier tube anodes; and in the conversion of the high impedance of the output tubes (s) of the amplifier to a suitable value for matching that of the reproducer. An example of an undesirable result of poor overall conversion efficiencyi is that the life of batteries employed in portable phonographs is much less than what it I might be ideally. r

It is an object of the present invention to devise improved transducers capable of controlling the delivery of such large amounts of power to utilization devices, such as transducers, that the need for amplifiers as in' the prior art is very much reduced or eliminated. a

It is a further object of the present invention to devise improved transducers capable of controlling the delivery of large amounts of power directly to a low impedance load, such as a reproducer, without the need of an impedance transformer.

In general, these objects have been attained by devising a transducer which controls the current in a utilization circuit by the use of a novel type of gas tube valve; The internal impedance of this valve has an extremely low average value I which isvariable over a usefully large range in response to microscopic mechanical movements such as those imparted to a phonograph stylus. As a result the transducer is able directly to modulate the flow of electrical energy through a closed load circuit between a low-voltage external source, such as a battery consisting of one or a few 1 /2 volt dry cells, and a low impedance reproducer. The low impedance of the gas tube valve result from the fact thatthe load current which it controls passes from its cathod to its anode not as an electron discharge across an insulating open space but as a conductioncurrent through a dense plasma.

The production of plasma is by an auxiliary electron space discharge and therefore is independent of the heavy electron conductionload current and of the output signal variations that occur in it during operation. Valving actionis effected by modulating a constriction inthe stream'of the auxiliary ionizing discharge to thereby modulate the plasmadensity and the se: ries resistance which it offers to the conduction current.

In the drawings: I r Figure 1 represents a longitudinal sectional view of one photograph pick-upembodiment of the present invention, The section is taken along'the line l-l of Figur 2 and in a plane which is vertical when the pick-up is in its operating position (restingon; a horizontalflat photograph record) I "Figure 2 representsanotherlongitudi'nal sectional view of the embodiment-of Figure 1', this section is taken along the line 22 of Figure -l and in a plane perpendicular to tion of Figure l; and I Figure 3 is a schematic circuit diagram of a sound reproducing system using the pick-up that of the sec shown in'Figures 1 and 2. I T

The pick-up H3 shown in Figures 1 and 2 comprises a gas-tight envelope H of substantially rectangular shape. Most of the envelope is made of rather thick' glass and is quite rigid. However,-its bottom [2 comprises a flexible, discshaped portion (disc l3) consisting of-a circular relatively thin metal diaphragm formed with a number of concentric corrugations and having its entire periphery fused to the edges ofan opening in the glass part of the bottom. 7 A rigid stylus I4 is carried in, and extends and is sealed through, the center of the disc l3. In this arrangement, and due to the flexibility of. the disc, transverse vibrations imparted to the needle point I 4a. on'the external end of the stylus M are imparted (with a reversal of direction) to its internal end. In other words, the center (I) of the disc'acts as a fulcrum for the stylus at a point intermediate its external and-internal ends.

The internal load current path through pickup extends between a main cathode l5 and a main anode l6 and in the operation of the pickup conduction over this path is through an extremely low impedance ionized gas plasma. In the novel gas valve employed herein, this plasma is established within the tube by ionizing its gaseous filling, G, by the action of an independent or auxiliary electron current flow. The auxiliary current flow is readily distinguishable from one load current flow through the tube. The following are two points of distinction:

The auxiliary flow includes as one component a space discharge electron current (as distinguished from conduction electron current) in which electrons attain sufficient velocities to ionize the gaseous filling, G, of the tube; and (2) the overall path of this space discharge is different from that of the load current, e. g., it starts at an emissive electron source other than that at which the load current path starts (this is the case herein where the two paths start at different cathodes); or it terminates at a collector other than the anode for the load current path; or it both starts and terminates at different electrodes than the internal load current path.

An auxiliary cathode 24 is mounted within the envelope II on the far side of the main cathode [5 from the main anode I6. It is surrounded by a cylindrical shield 25 having a narrow slot 21 on its side toward the main cathode 15. The shield causes the ion-producing electron discharge to be formed as a directed stream having a narrow cross-section, i. e., a constriction, in the region just beyond the point where the electrons emerge from the slot 21. With this type of construction, a plasma of very high density and therefore of very high conductivity, can be provided by an auxiliary electron discharge of surprisingly low current, i. e., it can be economically provided.

The shield 25 is supported on a rod 28 which in the example herein is fused through an end of envelope l l to serve as a terminal pin. This construction makes it possible to use a circuit in which one can control the bias between the auxiliary cathode 24 and the shield 25. This can be useful as a way of electrically varying the amount of constrictionproduced by the shield 25 without physically changing the width of its slot 21. However, it is entirely feasible to operate the auxiliary cathode and the shield at the same potential. Therefore, for some embodiments the shield 25 may simply be mounted by one or more rods which are secured to the interior of the envelope at suitable presses without necessarily extending any of the rods through to the outside of the envelope. In such an arrangement an internal inter-connection may be used for causing the shield and the auxiliary cathode to have the same potential. tensive experimentation, it is also feasible simply to allow this shield to float.

The slot 21 is formed between a lip I! which is an integral portion of the shield 25 and, as such, is stationary, and a movable shutter [8. The shutter I8 is mounted on the internal end of the stylus H in a position parallel to and somewhat spaced from the lip I1. It is apparent that in this arrangement a transverse Vibration of the stylus l4 will modulate the width of the slot 21. The pick-up device I!) shown herein may be processed in any of a number of ways Well known in'the art to provide a gaseous filling within its envelope prior to sealing off. Any suitable gas or mixture of gases may be utilized. The gas Moreover, as indicated by expressure for a particular pick-up will be in accordance with its specific electrode geometry and spacings and must be such as to favor the formation of a self-sustaining ionizing discharge. A number of gas tubes of the kind employed in the present pick-ups have been found to operate satisfactorily with a filling of helium at a pressure of approximately 750 microns. However, as is well known other gases and other pressures may be used, c. g., gas pressures which lie within the range between approximately microns and several millimeters of mercury.

One of a number of principles governing the operation of this pick-up can be best explained with reference to a common type of triode found in the class of prior art gas tubes known as thyratrons. This is a triode having a control grid which comprises an electron-opaque element, such as a metal plate, mounted cross-wise to the electron discharge path between the cathode and the anode and formed with a single aperture, such as asmall hole or a narrow slot, through which any electron discharge which takes place must pass. This grid is generally used only for controlling the anode-cathode potential at which this tube will fire. Its useis so restricted because of the fact that once the tube is ionized the grid loses control. That is, variations in its potential will cease to have any significant infiuence on the behavior of the tube. Since this is so, it is obvious that such variations will not be effective to control the anode-cathode current in the essential manner for using the tube as a class A amplifier, i. e., according to a substantially linear function of the instantaneous grid voltage.

It is of interest to consider some of the phenomena which occur within the tube. As soon as it fires, a positive-ion sheath immediately forms about all parts of the grid (if it is negatively biased) including the edges of its aperture. Thus, the effective size of the aperture is suddenly changed (reduced). Moreover, if the bias is then changed the sheath thickness will vary, and this will further change the size of the aperture (will further reduce it if the bias change is in the direction to increase its magnitude). Yet, it is observable that during the occurrence of all such changes the cathode-anode current nevertheless remains substantially constant. A phenomenon which may be responsible for this is also observable at the same time. It is that each change in the effective size of the grid aperture is accompanied by a change in the density of the plasma. Moreover, the same thing occurs if the changes in the aperture size are eifectedphysically (without dependence on the existence of a grid sheath and on inducing variations in the thickness thereof) Part of the method of the present invention is to use the very plasma-density variations, which apparently are responsible for preventing modulation of the auxiliary electron current, as a means for causing modulation of the main electron current. By so doing the ability to control the plasma density by changing the width of the slot 21 is directly translated into ability to control the low internal impedance of the load current path through the pick-up.

In the operation of this device, large currents of the order of one ampere per anode volt may circulate through the load circuit within a range of main-anode-to-main cathode potentials of between about zero volts and a saturating potential of about 1 to 2 volts. With a hard tube this would not be at all possible. One reason for this, is. that the emission from the cathode of a hard: tube is space-charge-limited' at low anode potentials. Moreover; suchcurrents. arenot even possible in a gas tube; (despite its freedom: from space-charge-limited cathode emissionlxif it. be of the prior art type: in which the ionizingelectron current moves between the. same electrodes as the load current; This is due to the fact that the current cannotlcontinuously exceed the: value atwhich the IR. drop in the external circuit will reducethe applied voltage below the value needed for sustaining ionization.

In the operation of the pick-up 1'0 its mainanode-to-main-cathode potential is always well below' the particular ionizing potential. Therefore, any electron space discharge which 'accom panics the electron conduction over'the load cur= rent path will occur at too low an'energy level to contribute to the production-of plasma and therefore its variations will not affect the plasma density. This will avoid the possibility of behavior in the'nature degeneration or regeneration; I

Figure 3 is a circuit diagram of 'a sound'reproducing system employing the pick-up H). "The utilization device is aloud speaker 30 having a voice coil 31 of a low impedance. such as a few ohms. The primary source of energy for driving the voice coil 3 I is alow'v'oltage battery 33'. This batteryshould be ahigh-current'source, such as a member of seris-parallel connected 1%; volt large size dry c'ells,"a'storagefbattery, or" the like The battery 33 is'connected' inseries with the voice coil 3i and the. load cur'rentpathof the pick-up. The source of energy for theauxiliary discharge'is a battery 34 which does not have to supply much current but must provide a suf ficient potential, e. g., -45 volts to ionize the gaseous filling, G.

Preferably the potential applied from battery 35 across the ionizing discharge path should only slightly exceed the ionizing potential since excessive potentials tend to favor the development of large amplitud plasma oscillations which, of course, would spuriouslymodulate the load current I a r A variable bias is represented at 35. This component is not essential for the circuit to be operative. However, it provides a convenient variable control-parameter which may be used. for example, as a volume control. Applying or increasing the negative bias to the shield will cause a sheath to form about it or thicken one already there to efiectively decrease the quiescent size of its slot 21, i. e., the size thereof for the rest position of the stylus M.

It will be noted that in this arrangement the main cathode l5 and the main anode l6 together constitute a composite anode for the internal auxiliary discharge originating at the auxiliary cathode 24. In the operation of the pick-up the main cathode I5 is copiously bathed in plasma with the result that even for extremely low main anode potentials (in fact for zero anode potential) the space charge of the electrons surrounding it appears to be completely neutralized. Under such conditions the load circuit currents have such magnitudes as toindicate internal impedances of the order of one or two ohms and in general appear to be limited only by the emission capabilities of the cathode. The current which can be observed to flow even for an anode potential of zero volts apparently is energized by the thermal velocities of the electrons emitted from the main cathode. That this is a correct 6: explanationtseems to beborne out by the fact that the anode currentfdoes not drop to zero until the anode p'otentialjismade. negative. to an extent corresponding tov the highest thermal-velocitiesof the emittedelectrons. I

"I claim: I

1'. A transducer comprising: a'sealed envelope containing a gaseous filling; within the envelope a main cathode and-a-main anode in spacedapart' relationship at the opposite ends of a-load current path for carrying through the transducer a conduction loadcurrent which may be accom panied by an electron discharge current, means forprod ucing an ionizing auxiliary electron discharge'over an auxiliary discharge path which does'not coincide over all ofit's'length with said load current path but hasa portion in proximity thereto to provide a conductive plasma between said anode and cathode; said last mentioned means comprising means; for producing a constriction in the path of the auxiliary discharge; and means responsive to mechanical forces ex: erted externally of said envelope for varying said constriction in correspondence therewith.

2. A, transducer comprising: a sealed envelope containing a gaseous filling; within the envelope a group of electrodesincluding a main cathode and. a. main anode for carrying aload current through the transducer; means forproducing an ionizing discharge ofielectrons along a path within, s'aid envelope which', ,tal en as a whole, is diiierent' than the path,taken as a whole, of said load current but has a portion in, proximity thereto to providea conductiveplasina between said main anode and main cathode, said means including, astwo "of its elements; anfemis'sive source of ionizing electrons'and a collectorthereof. at, mostfone of. said elementsjbeingincluded in, said'grouprof, electrodes, said "means further including ashield interposed between said source and said. collector of ionizing electrons and ha ing an aperture in alignment withthe discharge path thereof for producing a. constriction therein; a flexible portion for said envelopej a. movable element'positioried eiiectively to define aperimeter of said aperture and linked to said. flexible portion for responding to mechanical fiexings thereof to vary said constriction.

3. A transducer comprising: a sealed envelope containing a gaseous filling; within the envelope a main cathode and a main anode positioned to constitute a diode for carrying a load current through the transducer; means for producing an ionizing discharge of electrons, along a path within said envelope other than the path of the load current through said diode, to provide a conductive plasma between said main anode and main cathode, said means including an auxiliary cathode as a source of ionizing electrons and said main cathode as at least a portion of a collector of ionizing electrons; means including a shield interposed between said auxiliary cathode and said collector of ionizing electrons and having an aperture in alignment with said firstmentioned path for producing a constriction in the ionizing discharge; a flexible portion for said envelope; a movable element positioned effectively to define a perimeter of said aperture and linked to the inside of said flexible portion for responding to mechanical flexings thereof to vary said constriction.

4. A transducer comprising: a sealed envelope containing a gaseous filling; within the envelope a group of load circuit electrodes including a main cathode and a main anode having, respec- .of electrons from said auxiliary cathode to said collector over most of the possible lines of electron travel therebetween but having an opening so that it does not block their movement over other possible lines falling within said different path; a flexible portion for said envelope; a lever sealed through said flexible portion to extend I from the interior of said envelope to the exterior whereby. a movement imparted to the external portion of the lever will be reproduced at its internal portion; and means within the envelope for responding to movements of said lever to vary the density of said plasma and thereby the conduction which it provides in the load current path, said means including a movable element which is mechanically linked to the internal portion of said lever and is positioned to define a perimeter of said opening for responding to movements of the lever to vary the opening and thereby the constriction.

5. A transducer comprising a gas filled envelope containing a main cathode and a main anode having surfaces defining the ends of a load current path; means, including at least two elements to act respectively as a source of ionizing electrons and a collector thereof, forproducing an ionizing discharge along a different path to produce a conductive plasma between said anode and cathode; said last mentioned means comprising means for producing a constrictionin the path of the ionizing discharge; and means responsive to mechanical forces exerted externally of said envelope for varying said constriction. I

6. A' transducer comprising: a sealed envelope containing a gaseous filling; a main cathode and a main anode in said envelope for carrying a load current through the transducer; means for producing an ionizing discharge of electrons along a path within said envelope which, taken asa whole, is different than the path, taken as a whole, of said load current to provide a conductive plasma between said main anode and main cathode, said means including an emissive source of ionizing electrons and a collector thereof, said means further including a slotted shield interposed between said source and said collector of ionizing electrons with its slot positioned to constrict the ionizing discharge, said envelope having a flexible portion; a movable element linked to the inside of said flexible portion and positioned to respond to mechanical flexings thereof to alter the effective cross-sectional area of said slot.

7. A transducer including an envelope having a gaseous mediumtherein, a main cathode and a main anode within said envelope for carrying a load current through said transducer, auxiliary cathode means providing ionizing electrons directed toward said main cathode, a shield interposed between said cathodes and having an aperture producing a constriction in the discharge from said auxiliary cathode, said envelope having a flexible portion, a movable element adjacent said aperture for varying the effective area thereof to vary the constriction in said ionizing discharge and connection between said flexible portions of .said envelope and said movable element whereby said movable element responds to a flexing of said flexible portion ofsaid envelope.

EDWARD O. JOHNSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,507,884- Engler Sept. 9, 1924 1,871,253 Bauer Aug. 9, 1932 

